Reversal of stress-induced pregnancy blocks in mice by progesterone and metyrapone

It is established that diverse psychological stressors administered in the first trimester of pregnancy can cause the pregnancy to fail. The physiological mediators of this phenomenon are unknown. In Experiment 1, two strains of female mice were inseminated and randomly assigned to either first trimester physical restraint, restraint plus progesterone, or control conditions. Restrained females produced fewer pregnancies than did controls, and a reversal of this block through the daily administration of 500 micrograms progesterone was seen in HS but not C57 mice. Experiment 2 demonstrated that exposure to a predator will also block pregnancy in the first trimester in C57 mice, and that this can also be counteracted with progesterone administration. Experiment 3 showed that metyrapone, a drug which blocks the conversion of progesterone to corticosterone, also partially reverses a restraint-induced pregnancy block. These results suggest that lowered progesterone levels may be involved in pregnancy blocks induced by psychological stress.